Production of improved preimpregnated material comprising a particulate thermosetting resin suitable for use in the formation of a substantially void-free fiber-reinforced composite article

ABSTRACT

An improved multifilamentary fibrous material is formed having solid particles of a matrix-forming thermosetting resin substantially uniformly dispersed among adjoining filaments in the absence of fusion bonding. The thermosetting resin particles initially are dispersed in an aqueous medium containing an effective amount of a dissolved polymeric binding agent and the viscosity of the medium subsequently is substantially increased to at least 50,000 cps. to form a gelled impregnation bath having a plastic flow characteristic with shear-thinning behavior wherein the thermosetting resin particles are substantially uniformly suspended. The resulting impregnation bath is caused to flow between the adjoining filaments of the multifilamentary fibrous material with a concomitant viscosity reduction which aids in the incorporation of the resin particles. The concentration of the aqueous medium in the product is controlled to yield a uniform, handleable, drapable, tacky, and highly stable product. Upon the application of heat and pressure the improved product can be transformed into a fiber-reinforced composite article wherein the thermosetting resin forms the matrix phase.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation-in-Part Application of U.S. Ser. No. 928,917,filed Nov. 7, 1986 (now abandoned), of Robert Dyksterhouse and Joel A.Dyksterhouse entitled "Method and Apparatus for Impregnating the Tow toForm a Drapable Fibrous Material".

BACKGROUND OF THE INVENTION

Numerous techniques have been proposed in the past for the formation ofcomposite articles wherein fiber reinforcement is provided within amatrix of a polymeric material. Heretofore such composite articlescommonly have been prepared wherein the matrix is a highly cross-linkedthermosetting resin. During the formation of such composite articles thefibrous material commonly is impregnated with a liquid comprising theneat or dissolved uncured or partially-cured thermosetting resin to forma pliable, tacky prepreg which is subsequently placed in the desiredconfiguration and is cured to a predetermined solid form over anextended period of time. The drapable and tacky nature of such prepregshas greatly aided in their use when forming composite articles having acomplex configuration since adjoining plies tend to adhere well to eachother and stay in place during the composite formation process. Suchresulting thermosetting prepregs commonly must be used promptlyfollowing their manufacture or stored under refrigeration so as toeliminate problems resulting from the premature curing of the same.

When attempts have been made to dissolve thermosetting resins in asolvent and to impregnate a fibrous material followed by evaporation ofthe solvent, difficulties commonly have been encountered. For instance,some thermosetting resins are not readily soluble thereby making uniformimpregnation difficult to achieve. Complete solvent removal commonly hasbeen a problem and contributes to void formation in the resultingcomposite article. Also, the solvent may be injurious to workers in thearea unless special and often costly procedures are used to provide therequisite worker protection.

Blends of reinforcing fibers and thermoplastic fibers have been proposedfor use in the formation of composite articles wherein the thermoplasticfibers are melted to form the matrix phase. Such blends inherently lacktack which is useful during layup to form a fiber-reinforced compositearticle.

Reference also can be found in the literature to providing thematrix-forming thermosetting resin or thermoplastic polymer as smallsolid particles which are mixed with the fibrous material prior tocomposite formation. Heretofore, such proposals have not become acommercial reality primarily because of the inability to achieve uniformimpregnation of the fibrous reinforcement, the tendency of the particlesto further segregate within the fibrous material and to separate fromthe fibrous material, and the necessity to melt the particles followingplacement among the fibers so as to immobilize the same. Such fusion hasresulted in the formation of a stiff boardly product which lacks tackand is largely unsuitable for use in the formation of a compositearticle having a complex configuration. Also, when the particleimpregnation is not uniform, the matrix will not be uniformly dispersedamong fibers in the resulting composite article. This will result in avoid product having resin-rich and resin-lean areas and unpredictablenon-uniform mechanical properties.

British Pat. No. 1,264,432 concerns the application of a dispersion ofparticles of a thermoplastic polymer to newly spun glass fibers.

British Pat. No. 1,424,168 concerns the formation of a stiff sheet ortape prepreg wherein fibers are contacted with a bath containing water,thickening agent, and thermosetting resin or thermoplastic polymerparticles which immediately thereafter are melted at a temperaturebetween 60° to 100° C. to cause the particles to adhere to the fibers.Uniform fiber impregnation would not be achieved and the stable tackyand pliable product of the present invention would not result.

U.S. Pat. No. 4,292,105 concerns the impregnation of a fibrous materialfrom a bath of the specified composition containing water, thickener,and thermosetting resin or thermoplastic polymer particles. Theconditions described would not achieve uniform impregnation to produce aquality prepreg. Also, in the working examples the product would have nodrapability since it was heated to fuse the polymer.

U.S Pat. No. 4,626,306 concerns the impregnation of a fibrous lap withparticles of a thermosetting resin or thermoplastic polymer by dippingin a bath containing the particles in the absence of a binding agent.Uniform fiber impregnation would not be achieved and the stable tackyand pliable product of the present invention would not result.

U.S. Pat. No. 4,680,224 makes reference to the impregnation of fiberstrands with a poly(arylene sulfide) powder or a slurry of such powder.The patent is devoid of teachings concerning how such impregnation canbe accomplished in the improved manner contemplated herein or how onecould provide the improved product of the present invention.

Copending U.S. Ser. No. 114,362, filed Nov. 4, 1987, of RobertDyksterhouse and Joel A. Dyksterhouse is entitled "Production ofImproved Preimpregnated Material Comprising A Particulate ThermoplasticPolymer Suitable For Use In The Formation Of A Substantially Void-FreeFiber-Reinforced Composite Article", and sets forth a technique forforming a tacky and drapable fibrous material which uniformlyincorporates particles of thermoplastic polymer.

Copending U.S. Ser. No. 147,153, filed Feb. 5, 1988, of Alan C.Handermann and Edward D. Western is entitled "Improvements in theFormation of Preimpregnated Material Comprising ParticulateThermoplastic Polymer Suitable for Use in the Production of aSubstantially Void-Free Fiber-Reinforced Composite Article HavingImproved Transverse Properties", and sets forth a technique forimproving the transverse properties of the resulting composite articlewhich comprises a matrix of a thermoplastic polymer.

It is an object of the present invention to provide a method for theformation of an improved preimpregnated fibrous material suitable forthe formation of a substantially void-free thermoset composite articlecomprising a plurality of adjoining substantially parallel reinforcingfibers.

It is an object of the present invention to provide an improved methodfor impregnating a fibrous material with solid particles of amatrix-forming thermosetting resin in a uniform and consistentlyreliable manner.

It is an object of the present invention to provide an improved methodfor producing a preimpregnated fibrous product suitable for use in theproduction of composite articles having solid particles ofmatrix-forming thermosetting resin substantially uniformly dispersedbetween adjoining filaments in a uniform and stable manner in theabsence of fusion bonding.

It is an object of the present invention to provide a method forproducing an improved preimpregnated product containing solid particlesof thermosetting resin substantially uniformly dispersed among adjoiningfilaments which is drapable and tacky at ambient conditions, ishandleable without segregation of the particles within the fibrousmaterial, and which upon the application of heat and pressure can betransformed into a substantially void-free thermoset composite articleof a predetermined configuration.

It is an object of the present invention to provide an improved methodfor producing a preimpregnated fibrous material suitable for use in theproduction of a composite article wherein reinforcing fibers areprovided in a matrix of a thermoset resin with no solvent being presentwhen the resin it is introduced among the reinforcing fibers.

It is an object of the present invention to provide an improvedpreimpregnated fibrous material suitable for use in the formation of acomposite article comprising a fiber-reinforced thermoset resin which inpreferred embodiments is capable of being substantially fully cured on amore expeditious basis.

It is an object of the present invention to provide an improvedimpregnated fibrous material suitable for use in the formation of acomposite article comprising a fiber-reinforced thermoset resin which inpreferred embodiments is capable of being substantially fully cured inthe substantial absence of the generation of a volatile by-product.

It is an object of the present invention to provide an improvedpreimpregnated fibrous material which exhibits an extended shelf life atambient conditions in the absence of refrigeration and is suitable foruse in the formation of a fiber-reinforced thermoset resin.

It is a another object of the present invention to provide an improvedpreimpregnated fibrous material suitable for use in the formation of afiber-reinforced substantially void-free composite article and whichexhibits a combination of highly desirable characteristics as discussedherein including drapability, handleability without adverseconsequences, and tack.

It is a further object of the present invention to provide an improvedpreimpregnated fibrous material suitable for use in the formation ofhigh performance fiber-reinforced parts for use in aircraft, spacecraft,industrial machinery and automotive applications.

These and other objects, as well as the scope, nature and utilization ofthe present invention, will be apparent to those skilled in the art fromthe following detailed description and appended claims.

SUMMARY OF THE INVENTION

It has been found that a method for the production of an improvedfibrous material suitable for the formation of a substantially void-freecomposite article comprising a plurality of adjoining substantiallyparallel reinforcing filaments (e.g., a single end, a plurality of ends,a cloth, etc.) in association with a matrix-forming thermosetting resincomprises:

(a) preparing a dispersion of solid particles of a thermosetting resinin an aqueous medium which contains an effective amount of a dissolvedpolymeric binding agent (preferably a polyacrylic acid binding agentpossessing a cross linked molecular structure),

(b) substantially increasing the viscosity of the dispersion to form animproved impregnation bath whereby the viscosity of the resulting bathbecomes at least 50,000 cps. and the impregnation bath has a plasticflow characteristic with shear-thinning behavior which is sufficient tosubstantially uniformly suspend the particulate thermosetting resinwithin the bath,

(c) impregnating the adjoining substantially parallel reinforcingfilaments with the bath under conditions wherein the bath is caused toflow between the adjoining filaments by the application of work whereinthe bath flow inherently results in a reduction of the viscosity of thebath which aids in the incorporation of the particulate thermosettingresin between adjoining filaments, and

(d) controlling the content of the aqueous medium in the resultingfibrous material to provide a product having the particles ofmatrix-forming thermosetting resin substantially uniformly dispersedbetween adjoining filaments which inherently (1) is drapable and tackyat ambient conditions, (2) is handleable without segregation of theparticles within the fibrous material, and (3) which upon theapplication of heat and pressure can be transformed into a substantiallyvoid-free fiber-reinforced composite article of a predeterminedconfiguration wherein the thermosetting resin becomes substantiallycompletely cured and forms the matrix phase.

It has been found that a method for the production of an improvedfibrous material suitable for the formation of a substantially void-freecomposite article comprising a plurality of adjoining substantiallyparallel reinforcing filaments in association with a matrix-formingthermosetting resin comprises:

(a) providing a plurality of reinforcing fibrous tows each comprising aplurality of adjoining substantially parallel filaments,

(b) preparing a dispersion of solid particles of thermosetting resin inan aqueous medium which contains an effective amount of dissolvedpolyacrylic acid binding agent possessing a cross-linked molecularstructure,

(c) raising the pH of the aqueous medium (preferably through theaddition of ammonia or an alkyl amine having a boiling point less than100° C.) to form an improved impregnation bath wherein the viscosity ofthe resulting bath is substantially increased to at least 50,000 cps.through the stiffening of the molecules of the binding agent and theimpregnation bath has a plastic flow characteristic with shear-thinningbehavior which is sufficient to substantially uniformly suspend theparticulate thermosetting resin within the bath,

(d) situating the resulting bath within an impregnation apparatus,

(e) aligning the reinforcing fibrous tows in a side-by-side relationshipto form a substantially uniform sheet-like tape,

(f) feeding the sheet-like tape to the impregnation apparatus,

(g) impregnating the substantially uniform sheet-like tape with the bathwhile present in the impregnation apparatus under conditions wherein thebath is caused to flow between the adjoining filaments of the sheet-liketape by the application of work wherein the flow inherently results in areduction of the viscosity of the bath which aids in the incorporationof the particulate thermosetting resin between adjoining filaments, and

(h) controlling the content of the aqueous medium in the resultingsheet-like tape to provide a product having the particles of thematrix-forming thermosetting resin substantially uniformly dispersedbetween adjoining filaments in the absence of fusion bonding whichinherently (1) is drapable and tacky at ambient conditions, (2) ishandleable without segregation of the particles, and (3) which upon theapplication of heat and pressure can be transformed into a substantiallyvoid-free fiber-reinforced composite article of a predeterminedconfiguration wherein the thermosetting resin becomes substantiallycompletely cured and forms the matrix phase.

An improved preimpregnated fibrous material suitable for the formationof a fiber-reinforced composite article is provided which comprises (a)a plurality of adjoining substantially parallel reinforcing filaments,(b) an effective amount of a polymeric water-soluble binding agent(preferably a polyacrylic acid binding agent possessing a cross-linkedmolecular structure wherein the molecules are extended), (c) aqueousmedium, (d) and solid particles of thermosetting resin substantiallyuniformly dispersed between adjoining filaments in the absence of fusionbonding, which inherently (1) is drapable and tacky at ambientconditions, (2) is handleable without segregation of the particleswithin the fibrous material, and (3) which upon the application of heatand pressure can be transformed into a substantially void-free compositearticle of a predetermined configuration wherein the thermosetting resinbecomes substantially completely cured and forms the matrix phase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic overall view of a preferred apparatus forimpregnating a plurality of adjoining tows in accordance with theimproved process of the present invention;

FIG. 2 is a top view of a portion of the apparatus of FIG. 1, takenalong lines 2--2;

FIG. 3 is a perspective view of the board for mounting spools of fibroustows to be impregnated in the apparatus of FIG. 1;

FIG. 4 is a partial sectional view of a tensioning apparatus utilized inthe initial portion of the apparatus shown in FIG. 1;

FIG. 5 is a sectional view of a portion of the apparatus used to alignthe fibrous tows as they pass through the apparatus of FIG. 1;

FIG. 6 is a sectional view of another portion of the aligning means ofthe apparatus shown in FIG. 1;

FIG. 7 is a partial sectional view of the grooved rollers and eccentricrollers utilized in a portion of the apparatus in FIG. 1;

FIG. 8 is a cross-sectional view of the apparatus shown in FIG. 7;

FIG. 9 is a cross-sectional view of the apparatus shown in FIG. 8 withthe eccentric roller rotated 180°;

FIG. 10 is a sectional view of the apparatus of FIG. 9 taken along lines10--10;

FIG. 11 is a perspective view of the impregnation section shown in FIG.1;

FIG. 12 is a side sectional view of the apparatus shown in FIG. 11 takenalong lines 12--12;

FIG. 13 is an alternative arrangement for impregnating the fibrous towwhich could be substituted for that shown in FIG. 11;

FIG. 14 is an alternative arrangement for the impregnation section whichcould be substituted for that shown in FIG. 11;

FIG. 15 is a perspective view of the die section shown in FIG. 1;

FIG. 16 is a cross-sectional view taken along line 16--16 of FIG. 15;

FIG. 17 is a side sectional view of a portion of the rollers shown onthe left side bottom portion of FIG. 1;

FIG. 18 is a rear view of the rollers of FIG. 17 shown along line 18--18thereof;

FIG. 19 is a perspective view of the drying apparatus shown in FIG. 1;

FIG. 20 is a perspective view of the take-up apparatus shown in FIG. 1;

FIG. 21 is a schematic side sectional view of a preferred apparatus forimpregnating cloth;

FIG. 22 is a schematic exploded view of the formation of a laminateresulting from the impregnated fibrous material of the presentinvention;

FIG. 23 illustrates the closing of the die about the laminated materialwhen forming a composite article of a predetermined configuration;

FIG. 24 is a side view of the resulting composite article;

FIG. 25 is a schematic drawing of another preferred apparatus forimpregnating tows in accordance with the improved process of the presentinvention;

FIG. 26 is a top view of an immersion means useful in the presentinvention;

FIG. 27 is a front, partially sectional view of the apparatus of FIG. 26taken along line 30--30 of FIG. 26;

FIG. 28 is a side sectional view of the immersion means useful in thepresent invention; and

FIG. 29 is a sectional view of the immersion means useful in the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A fibrous material comprising a plurality of adjoining substantiallyparallel filaments initially is selected for use as fibrousreinforcement in the present invention. Such fibrous material may beprovided as a single multifilamentary end, a plurality ofmultifilamentary ends each comprising a plurality of substantiallyparallel filaments, a cloth (e.g., woven, knitted, or braided) whichincorporates a plurality of substantially parallel filaments, etc. Forinstance, fibrous tows or tapes of varying widths conveniently may beselected as the reinforcing fibrous material. A single tow of arelatively narrow width may be impregnated. Alternatively, wider fibrousmaterials having widths of 1 to 48 inches or more likewise may undergosuch impregnation.

Those fibrous materials which heretofore have been used as fibrousreinforcement in the production of composite articles commonly areselected. For instance, representative fibrous materials include carbon,glass, aramid, silicon carbide, silicon nitride, boron nitride, othersynthetic polymers capable of use at elevated temperatures, mixtures ofthese, etc. The preferred carbonaceous fibrous materials contain atleast 90 percent carbon by weight which may comprise carbon which iseither amorphous or graphitic in nature. Preferred carbonaceous fibrousmaterials commonly contain at least 3,000 (e.g., 3,000 to 12,000, ormore) substantially parallel filaments per end. Such carbon filamentscommonly are approximately 4 to 10 microns in diameter. Representativepreferred carbonaceous fibrous materials for use in connection with thepresent invention are Celion carbon fibers which are commerciallyavailable from BASF Structural Materials, Inc., Charlotte, N.C., U.S.A.Also, glass filaments which are capable of being readily impregnated bythe flow of the impregnation bath described hereafter may be selected.Such glass filaments commonly have a diameter of approximately 10 to 20microns (e.g., 12 to 13 microns). Suitable aramid fibers arecommercially available from E.I. DuPont, Wilmington, Del., U.S.A., underthe Kevlar trademark and may have a diameter as low as 5 to 8 microns.The fibrous material preferably is unsized, or if a size is present suchsize does not preclude the ready insertion of the thermosetting resinparticles between adjoining filaments during the impregnation step(described hereafter). It is, of course, important that the reinforcingfibers selected well retain their fibrous integrity and reinforcingproperties at the temperature used to consolidate and cure thethermosetting resin particles while in association with such fibersduring composite formation. For instance, in a preferred embodiment thefibrous reinforcement can withstand temperatures greater than 537.8° C.(i.e., 1000° F.).

During the preparation of the impregnation bath employed in the processof the present invention, small solid particles of a matrix-formingthermosetting resin initially are dispersed in an aqueous mediumcontaining a dissolved polymeric binding agent and having a relativelylow viscosity. Thereafter the viscosity of the dispersion issubstantially increased to form a stable improved impregnation bathhaving a plastic flow characteristic with shear-thinning behavior (asdescribed hereafter).

The thermosetting resin which is selected for use in the presentinvention must be capable of being placed in a powder form (i.e., assmall solid particles at ambient conditions while in a less than a fullycured state) and be capable of forming a substantially void-free matrixwhen heated under pressure to accomplish consolidation and curing. Whenheated at an appropriate temperature such particles of thermosettingresin will either completely or partially melt or otherwise be renderedpliable and/or heat-sinterable at their adjoining surfaces so as to formthe predetermined configuration of the composite article. It ispreferred that the solid particles of thermosetting resin be capable ofundergoing complete melting when heated and that such melting take placeat a temperature below that at which substantial additional crosslinkingtakes place. The substantially complete curing of the thermosettingresin when heated under pressure (preferably at a higher temperature)may take place via a condensation reaction, an addition reaction, orcombination of condensation and addition reactions. Conventional curingagents, hardeners, or other agents designed to initiate or promote thedesired crosslinking may also be present. If the curing takes place viaa condensation reaction with the evolution of gaseous by-products, meansare provided to accommodate the removal of such volatiles (i.e., an openmold or a vacuum bag procedure) during at least a portion of thecomposite formation step (as described hereafter). In a particularlypreferred embodiment, a thermosetting resin is selected which curesduring at least the final portion of the composite formation step via anaddition reaction without the evolution of a gaseous product. The use ofa thermosetting resin which undergoes such addition reaction duringcuring simplifies the achievement of the desired substantially void-freecomposite article when the preimpregnated fibrous material of thepresent invention is transformed.

It is preferred that the thermosetting resin exhibits a continuous usetemperature of at least 25° C., and most preferably a continuous usetemperature of at least 90° C. Such continuous use temperature isdetermined by measuring the fully moisture saturated glass transitiontemperature of the substantially completely cured thermosetting resin.

Representative solid particulate resins for use when practicing thepresent invention include thermosetting resins selected from among thephenolic resins, polyester resins, melamine-formaldehyde resins,urea-formaldehyde resins, casein-formaldehyde resins, polyimide resins,polyurethane resins, epoxy resins, diallyl phthalate resins, vinyl esterresins, polybutadiene(1,2) resins, cyanate ester resins, cyanamideresins, etc. The preferred thermosetting resins for use in the presentinvention are the polyimide resins, epoxy resins, and cyanate esterresins.

The phenolic resins commonly are formed by the reaction of phenol andformaldehyde and may be of either the resole or novolac types.Representative commercially available phenolic resins which may beprovided in solid particulate form are the PLENCO 07200 moldingcompounds available from Plastic Engineering Co. of Sheboygan, Wis.,U.S.A., and DUREZ 17080 molding compound available from OccidentalChemical of North Tonawanda, N.Y., U.S.A.

The polyester resins commonly are either alkyed resins or unsaturatedpolyester resins. The alkyed resins are formed by the reaction ofsaturated dibasic acids with polyhydroxy compounds. The unsaturatedpolyester resins are formed by the reaction of dibasic acids (e.g.,fumaric acid) or anhydrides (e.g., maleic anhydride) which are partiallyor completely composed of 1,2-ethylenically unsaturated monomer unitswith dihydric alcohols. During such formation the resulting polymercommonly is dissolved in a reactive vinyl monomer, such as styrene,vinyl toluene, diallyl phthalate, or methyl methacrylate. The additionof a free radical initiator such as an organic peroxide results in across-linking reaction between the unsaturated polymer and theunsaturated monomer. Representative commercially available alkyed resinswhich may be provided in solid particulate form together with theirsource are the GLASKYD 2000-4000 series of molding compounds availablefrom American Cyanamid of Perrysville, Ohio, U.S.A. A representativecommercially available unsaturated polyester resin is PALATAL HT resinwhich has been partially cured to become a solid at ambient temperature.Such unsaturated polyester resin is commercially available from BASFAktiengesellschaft of Ludwigshafen, West Germany.

The melamine-formaldehyde, urea-formaldehyde, and casein-formaldehyderesins frequently are termed amino resins. These commonly are formed bythe condensation of melamine, urea or casein with formaldehyde resultingin a cross-linked resin. Representative melamine resins which may beprovided in solid particulate form are the PERSTORP 791-796 resins whichare available from Perstorp, Inc. of London, England.

The polyimide thermosetting resins are preferred for use in the presentinvention and include the poly(bismaleimide) resins. These may beprovided as crosslinkable homopolymers, copolymers, or terpolymers, andare commonly produced by the reaction of an aromatic dianhydride with anaromatic diamine. Particularly preferred high use temperature polyimidesare of the reverse Diels-Alder (RDA) polyimides. During the formation ofthe RDA polyimides, amide formation first takes place. This is followedby imidization and an irreversible Diels-Alder reaction, and acombination via an addition reaction to form a stable cross-linkedpolyimide. Accordingly, the preferred RDA polyimide resins aresubstantially fully imidized and are capable of undergoing an additioncross-linking reaction in the substantial absence of the generation ofvolatile by-products. Fully imidized PMR-15 polyimide is commerciallyavailable from Hysol Grafil Composite Components Co. of Cleveland, Ohio,U.S.A. and is an example of a particularly preferred RDA polyimide foruse in the present invention. The fully imidized PMR-15 polyimide isprepared from three monomers: monomethylester of5-nornborne-2,3-dicarboxylic acid, dimethyl ester of3,3',4,4'-benzophenone tetracarboxylic acid, and 4,4'-methylenedianiline in the mole ratio 2:2.087:3.087. Another representativecommercially available thermosetting polyimide is THERMID 1P-600 resinavailable from National Starch and Chemical Company of Bridgewater,N.J., U.S.A.

The poly(bismaleimide) resins undergo polymerization by reaction of themaleimide double bond with another unsaturated system or by the Michaeladdition of nucleophylic species at relatively low temperature withoutthe evolution of volatile by-products. Such poly(bismaleimide) resinsmay be modified in the sense that usual comonomers, such as vinyl orallyl compounds or diamino or aminophenal compounds, are added. Apreferred poly(bismaleimide) resin is modified 5250-2 poly(bismaleimide)resin supplied by BASF Aktiengesellschaft of Ludwigshafen, West Germany.Other representative poly(bismaleimide) resins are KERIMID resinavailable from Rhone-Poulenc Inc. of Monmouth Junction, N.J., U.S.A. orRhone-Poulenc S.A. of Paris, France, and the Compimide resins availablefrom Technochemie GmbH of West Germany.

Heretofore, thermosetting polyimide resins commonly have been introducedto the fibrous reinforcement while dissolved in a solvent. Accordingly,the present invention offers significant processing advantages sincethere is no need to remove and handle a solvent which in at least someinstances may pose a health hazard when practicing the concept of thepresent invention. Such solvent removal in the prior art commonly istime consuming, requires the use of special equipment, and requires theuse of special procedures to protect workers in the area.

The polyurethane resins are formed by the reaction of diisocyanates withpolyols, polyamides, alkyed polymers, and polyether polymers.Representative commercially available polyurethane resins which may beprovided in solid particulate form are the US0050, 60, 70 and 80 seriesresins available from Dexter Co. of Arlington, Tex., U.S.A.

The epoxy resins contain epoxide groups and commonly are curable byreaction with an appropriate curing agent such as an amine, alcohol,phenol, carboxylic acid, acid anhydride, or mercaptan. Brominated epoxyresins, cycloaliphatic epoxy resins and epoxyimide resins are includedin the epoxy classification. Representative commercially available epoxyresins which may be provided in solid particulate form are Rigidite 5208resin available from NARMCO of Anaheim, Calif., U.S.A. which has beenpartially cured to become a solid at ambient temperature; Epon 1000 and2000 resins available from Shell Chemical Co. of Houston, Tex., U.S.A.;and MH19F-0217 novolac epoxy resin available from Dexter Co. ofArlington, Tex., U.S.A.

The diallyl phthalate resins commonly are supplied as prepolymers suchas diallyl phthalate or diallyl isophthalate and commonly are cured byperoxides. Other variations include trifunctional diallyl maleate,triallylcyanurate, and allyl carbonate. Representative commerciallyavailable diallyl phthalate resins which may be provided in solidparticulate form are the DAP molding compounds available from CosmicPlastics of San Fernando, Calif., U.S.A.

The vinyl ester resins commonly are esters of acrylic acid and/ormethacrylic acid. They are often derived from an epoxy resin.Frequently, the curing is accomplished via the polymerization ofunsaturation. Representative commercially available vinyl ester resinswhich may be provided in solid particulate form are PALATAL V7519 resinavailable from BASF Aktiengesellschaft of Ludwigshafen, West Germany,which has been partially cured to become a solid at ambient temperature.

Polybutadiene(1,2) resins commonly are cross-linked by means of thependant vinyl group in the prepolymer stage. The hardness and cross-linkdensity is a function of the severity at the post cure time andtemperature.

The cyanate ester and cyanamide resins were introduced during the early1970's and are formed when the monomers essentially trimerize via anaddition reaction to form thermosetting modified triazine structures.Representative commercially available cyanate ester resins which may beprovided in solid particulate form are those of the BT series availablefrom Mitsubishi Gas Company of Japan.

The matrix-forming thermosetting resin preferably is provided as smallsolid particles having a particle size in the range of approximately 0.1to 100 microns, and most preferably a particle size of 0.1 to 20microns. In a particularly preferred embodiment at least 50 percent ofthe particles are smaller than 10 microns. Accordingly, the meanparticle size preferably is less than 50 microns. Those thermosettingresins which melt with difficulty or are merely heat-sinterable areprovided as extremely small particles. Cryogenic grinding or other knowngrinding or particle-forming techniques may be employed to provide thethermosetting resin in an extremely fine powder form.

The aqueous medium in which the solid particles of thermosetting resinare dispersed contains water as the major component and optionally mayinclude in a minor concentration one or more water-miscible organicliquids which do not interfere with the desired uniform impregnation(e.g., methanol, ethanol, isopropanol, ethylene glycol, etc.). In someinstances, the presence of such water-miscible organic liquids, becauseof their increased volatility, may expedite the removal of a portion ofthe aqueous medium following impregnation, should this be desired.However, in a preferred embodiment the aqueous medium is substantiallypure water.

A suitable polymeric binding agent is dissolved in an effectiveconcentration in the aqueous medium in which the solid particles ofthermosetting resin are initially dispersed while at a relatively lowviscosity. Such binding agent must be capable of facilitating asubstantial increase in the viscosity of the dispersion during asubsequent step of the process to form an improved highly stable gelledimpregnation bath having a plastic flow characteristic withshear-thinning behavior wherein the polymer particles are substantiallyuniformly suspended. Such increase in viscosity commonly is accomplishedthrough the addition of an agent which interacts with the dissolvedpolymeric binding agent. The resulting impregnation bath containing thedissolved polymeric binding agent, when caused to flow between adjoiningfilaments of the fibrous material by the application of work, exhibits ashear-thinning behavior which inherently results in a reduction of theimpregnation bath viscosity and thereby greatly aids in theincorporation of the solid particulate thermosetting resin betweenadjoining filaments. The binding agent when present in the resultingimpregnation bath also causes the filaments to adhere well to each otherand to exhibit tacky properties in the presence of the aqueous medium.

The preferred water-soluble polymeric binding agent which possesses therequisite properties for use in the present invention is a polyacrylicacid binding agent possessing a cross-linked molecular structure. Suchbinding agent is a water-soluble carboxy vinyl polymer (sometimes termedcarboxy polymethylene) of the following basic structure: ##STR1## Thecrosslinks are slight in nature and commonly are provided by polyalkenylpolyether at a level which allows for water solubility. When heretoforeused in cosmetic products, this material has been known as "carbomer".This polymeric binding agent is commercially available from B. F.Goodrich, Cleveland, Ohio, U.S.A. under the Carbopol trademark, and isdescribed in its May, 1986 publication entitled "Carbopol® Water SolubleResins". Representative, water-soluble binding agents of this type whichare available from B. F. Goodrich are Carbopol 910, Carbopol 934,Carbopol 940, and Carbopol 941. The polyacrylic acid binding agentpossessing a cross-linked molecular structure advantageously may possessa molecular weight from 450,000 to 4,000,000, and preferably from750,000 to 3,000,000. Such binding agents commonly possess a coiledmolecular structure when purchased.

The water-soluble binding agents employed in the present inventioncommonly are provided in the dispersion prior to the substantialincrease in viscosity in a concentration of approximately 0.01 to 5percent (e.g., 1 to 5 percent) based upon the total weight of thedispersion, and preferably in a concentration of approximately 0.01 to 2percent by weight based upon the total weight of the dispersion. If theconcentration of the binding agent is too low, the desired subsequentsubstantial increase in viscosity will not be possible or the level oftack obtained will not be sufficient. Also, if the concentration ofbinding agent is too great, no concomitant advantage will be realizedand the binding agent may interfere with the desired compositeproperties. The higher molecular weight binding agents (as described)offer the advantage of functioning at lesser concentrations.Accordingly, in a particularly preferred embodiment the water-solublebinding agent is present in the initial dispersion in a concentration ofapproximately 0.1 to 1.5 percent by weight (e.g., in a concentration ofapproximately 0.1 to 0.8 percent by weight).

Other representative water-soluble polymeric binding agents suitable foruse in the present invention include polyvinyl alcohol and polyvinylpyrrolidone.

The particulate thermosetting resin commonly is provided in the initialdispersion containing the dissolved polymeric binding agent in aconcentration of approximately 5 to 50 percent by weight based upon thetotal weight of the dispersion, and preferably in a concentration ofapproximately 10 to 30 percent by weight (e.g., approximately 15 percentby weight).

The initial dispersion may optionally contain a surfactant in a minorconcentration to aid in the wetting of the particles of thethermosetting resin. For instance, the surfactant may be present in aconcentration of approximately 0.005 to 0.5 percent by weight of thetotal dispersion, and preferably in a concentration of 0.01 to 0.2percent by weight. Any surfactant may be employed provided it does notinterfere with the subsequent viscosity increase or otherwise adverselyinfluence the resulting product, such as through the creation of anexcessive amount of foam or bubbles. Representative surfactants whichmay be used to advantage in the present process include surfactantsbased on alkylaryl polyether alcohols (e.g., alkyl phenoxypolyethoxyethanol), sulfonates and sulfates. A surfactant commercially availablefrom the Rohm and Haas Company, Philadelphia, Penna., U.S.A., under theTriton X100 trademark, may be used to advantage.

A small amount of a lubricant such as glycerine may optionally beprovided in the initial dispersion. For instance, glycerine may beprovided in a concentration of approximately 2 percent by weight.However, good results are achieved in the total absence of suchlubricant.

A finely divided particulate filler may also be included in theimpregnation bath in order to modify the properties of the resultingcomposite article. Representative fillers include carbon powder;metallic powders, such as aluminum, titanium, etc.; silicates; silicon;tungsten carbide; porcelain; clay; feldspar; quartz; titanates; mica;glass beads; silica; etc.; and discontinuous fibers such as inorganic ororganic fibers. In a preferred embodiment such fillers are not employed.

The viscosity of initial dispersion is sufficiently low so that theresin particles can be thoroughly dispersed throughout the aqueousmedium while using moderate agitation. Particularly good results areobtained when both the polymeric binding agent and the particulatethermosetting resin are blended together as solid particles and arethereafter introduced into the aqueous medium with moderate agitation.Commonly, the viscosity of the initial dispersion is well below 50,000cps. when tested using a Rheometrics Stress Reometer (Model RSR/M) whileoperating at a shear rate of 0.01 reciprocal second Preferably, theviscosity of the initial dispersion is no more than 30,000 cps., andmost preferably no more than 20,000 cps. (e.g., 2,000 to 20,000 cps.) inaccordance with such test conditions. However, satisfactory results areachieved at greater viscosities when the initial dispersion exhibits asufficiently high shear-thinning behavior to enable good dispersion ofthe particles of thermosetting resin while being well agitated. See, forinstance, the process embodiments reported in the examples.

Once the dispersion of the particulate thermosetting resin is achievedunder such relatively low viscosity conditions, the viscosity issubstantially increased from the viscosity level initially exhibited toform an improved impregnation bath having a plastic flow characteristicwith shear-thinning behavior which is sufficient to substantiallyuniformly suspend the particulate thermosetting resin within the bath.More specifically, the viscosity of the dispersion is raised to at least50,000 cps. when tested using a Rheometrics Stress Rheometer at a shearrate of 0.01 reciprocal second. Such viscosity increase preferably is atleast 50 percent, and viscosity increases of 1.5 to 25 times, or more,may be utilized. Commonly, the resulting viscosity will be within therange of approximately 50,000 to 3,000,000 cps. when using a RheometricsStress Rheometer at a shear rate of 0.01 reciprocal second, andpreferably within the range of 50,000 to 1,000,000 cps. (e.g., 50,000 to250,000 cps.). Such viscosity increase also is accompanied by asignificant increase in the tackiness and binding properties of thebinding agent.

The viscosity commonly is caused to increase through the addition of anagent which interacts with the dissolved polymer binding agent. Suchinteraction in preferred embodiments may be accomplished through theadjustment of the pH of the initial dispersion. For instance, when adissolved polyacrylic acid binding agent possessing a cross-linkedmolecular structure is employed, the dispersion inherently exhibits anacidic pH which commonly falls within the range of approximately 2.5 to3.5. When a base is added to the initial dispersion, the correspondingadjustment in the pH causes the stiffening (i.e., uncoiling andextension) of the previously coiled molecules of such binding agentwhich substantially raises the viscosity of the dispersion with theparticles of thermosetting resin being substantially uniformly suspendedin a highly stable manner within the resulting impregnation bath. Forinstance, sufficient base may be added to the dispersion to accomplishsome neutralization and to raise the pH to within the range of 4 to 10,and most preferably within the range of 6 to 8. The neutralization stepalso results in a dramatic increase in the binding and tackifyingproperties.

The base used to stiffen the previously coiled molecules of the bindingagent may be sodium hydroxide. However, it has been found that improvedresults are achieved when the viscosity of the previously formeddispersion of thermosetting resin particles is substantially increasedthrough the addition of a base selected from the group consisting ofammonia, an alkyl amine, and mixtures of the foregoing. Representativealkyl amines suitable for use in the process of the present inventionhave a boiling point less than 100° C. and include methylamine,ethylamine, trimethylamine, and mixtures of these. The utilization of anammonia or alkyl amine base in the context of the present invention hasbeen found to make possible the formation of a composite article whichexhibits superior mechanical properties. The ammonia or alkyl aminepreferably is dissolved in an aqueous solvent when added to thepreviously formed dispersion of solid particles of a thermosetting resinin an aqueous medium which contains an effective amount of the dissolvedpolyacrylic acid binding agent (as described). In a particularlypreferred embodiment the ammonia and/or alkyl amine is provided in arelatively dilute concentration in water of approximately 5 to 20percent by weight when added to the dispersion of thermosetting resinparticles. Also, in a particularly preferred embodiment ammonia isdissolved in water to provide an ammonium hydroxide solution. Theviscosity of the dispersion is caused to increase as the ammonia and/oralkyl amine interacts with the dissolved polyacrylic acid binding agent(as described) to form an ammonium or alkyl ammonium polyacrylatebinding agent having stiffened molecules possessing a cross-linkedmolecular structure.

Alternatively, when the dissolved polymeric binding agent is polyvinylalcohol, borax (i.e.. Na₂ B₄ O₇.10H₂ O) may be added to bring out thedesired viscosity increase. When polyvinyl pyrrolidone is employed asthe polymeric binding agent, the initial dispersion may be simplyacidified whereby the pH is lowered in order to bring about the requiredsubstantial increase in viscosity.

The plastic flow behavior of the resulting impregnation bath providesthe particulate thermosetting resin in a vehicle which is capable ofbringing about the impregnation of the fibrous material in a highlyuniform manner over an extended period of time. The particulatethermosetting resin is well suspended therein, thereby making possiblethe formation of a consistent and uniform product.

In a preferred embodiment, the resulting impregnation bath possesses aplastic flow rheology characterized by a Brookfield Yield Value abovethe minimum value required to permanently suspend even the largestparticles of the thermosetting resin present in the static bath whileunder the influence of gravity. Such Minimum Brookfield Yield Value canbe theoretically calculated for any specific particle of thermosettingresin in accordance with the following equation:

    Minimum Brookfield Yield Value=[23.6 R(D-D.sub.o).sub.g ].sup.2/3

with units in dynes/cm.², where:

R=Particle Radius (cm.),

D=Particle Density (gm./cc.),

D_(o) =Suspending Medium Density (gm./cc.), and

g=Gravitational Constant=980 cm./sec.².

For example, the Minimum Brookfield Yield Value for a 50 micron particleof thermosetting resin (D=1.30 gm./cc.) is approximately 6.6 dynes/cm.².The Brookfield Yield Value for any dispersion can be determined withsufficient accuracy using a Brookfield RVT viscometer and spindle No. 1in accordance with the following equation: ##EQU1## In preferredembodiments the Brookfield Yield Value of the impregnation bath is atleast 1.5 times the Minimum Brookfield Yield Value, and most preferablyat least 2 times the Minimum Brookfield Yield Value (e.g., 2 to 10times, or more) in order to build further stability into the improvedimpregnation bath which is utilized in the process of the presentinvention.

Next, the adjoining substantially parallel reinforcing filaments areimpregnated with the improved impregnation bath under conditions whereinthe bath is caused to flow between adjoining filaments by theapplication of work wherein the bath flow inherently results in asubstantial reduction of the relatively high viscosity of the bath whichaids in the incorporation of the particulate thermosetting resin betweenadjoining filaments. Accordingly, the improved impregnation bathexhibits a shear-thinning behavior which is an important element of thepresent invention. For instance, a dispersion viscosity of 100,000 cps.using a Rheometrics Stress Rheometer (Model RSR/M) at a shear rate of0.01 reciprocal second for such an improved impregnation bath typicallywill be reduced to less than 500 cps. at a shear rate of 500 reciprocalseconds. This behavior permits the particulate thermosetting resin fromsettling while in the static bath at zero shear rate conditions andallows the particles of thermosetting resin to be impregnated betweenadjoining filaments when work is applied to the bath. Also, once thezero shear condition is reestablished within the resulting fibrousmaterial, a highly stable prepreg product results as discussed herein.

The improved impregnation bath may be caused to flow between theadjoining filaments of the fibrous material by any one of a number oftechniques. Preferably, the adjoining filaments are somewhat spread atthe time of impregnation. The impregnation may be carried out while thefibrous material is immersed in the impregnation bath. Work is appliedto the bath as the adjoining filaments while under tension are passed incontact with at least one solid member (e.g., a stationary rod orroller). Alternatively, the impregnation may be carried out as thefilaments contact the outer surface of at least one perforated tubethrough which the bath is forced, or the filaments while in contact withthe impregnation bath are passed through a die and/or between one ormore sets of rollers. If desired, the impregnation of the fibrousmaterial may be carried out immediately following fiber formation withthe fibers passing to the impregnation apparatus.

Next, the content of the aqueous medium in the resulting fibrousmaterial is controlled to provide a product having the particles ofmatrix-forming thermosetting resin substantially uniformly dispersedbetween adjoining filaments in the absence of fusion bonding. Followingimpregnation, the concentration of aqueous medium in the resultingfibrous material is controlled at the desired level. Commonly, suchaqueous medium within the fibrous material is controlled at aconcentration above approximately 40 percent by weight, and preferablywithin the range of approximately 10 to 70 percent by weight based uponthe total weight. In a particularly preferred embodiment, the aqueousmedium is provided at a concentration within the range of approximately40 to 60 percent by weight.

Commonly, the fibrous material following impregnation is dried underconditions wherein a portion of the aqueous medium is volatilized andthen is immediately used for composite formation, or is otherwise storedunder conditions wherein it is sealed and further loss of the aqueousmedium is minimized or prevented prior to composite formation.

In another embodiment of the process, the resulting fibrous materialfollowing drying to remove aqueous medium is subsequently contacted(e.g., sprayed) with additional aqueous medium in order to maintain thedesired characteristics. Accordingly, the quantity of aqueous mediumwithin the product can be reduced or increased at will to fine tune thecharacteristics of the resulting product to best meet the needs of aspecific end use.

Commonly, the resulting product which is suitable for the formation of asubstantially void-free composite article contains the particles ofmatrix-forming thermosetting resin in a concentration of approximately 6to 45 percent by weight, and preferably in a concentration ofapproximately 8 to 30 percent by weight (e.g., approximately 15 percentby weight). Commonly, the resulting product contains the fibrousmaterial in a concentration of approximately 15 to 55 percent by weight(e.g., approximately 20 to 30 percent by weight), and the water-solublebinding agent in a concentration of approximately 0.02 to 2.2 percent byweight (e.g., approximately 0.04 to 1.5 percent by weight).

The resulting product prior to composite formation is drapable atambient conditions and can readily be shaped in a manner similar to thatof a prepreg formed using an uncured or partially-cured thermosettingresin which is not in solid particulate form. Such drapable characterpreferably is evidenced by a flexural rigidity of less than 15,000 mg. .cm., and most preferably less than 10,000 mg..cm. (e.g., less than 5,000mg. . cm.) when tested in accordance with ASTM D1388. This enables theformation of a composite article by filament winding or a compositearticle wherein the impregnated fibrous material must assume a complexconfiguration within a mold. The tacky nature of the product can beattributed to the gelled nature of the impregnation bath. This enablesadjoining layers of the fibrous material to well adhere and to remain ata predetermined location during composite formation. In a preferredembodiment, the resulting fibrous material passes the tack test of NASATechnical Bulletin 1142. Also, the product prior to composite formationis highly stable and handleable without segregation of the particleswithin the fibrous material. Such absence of segregation leads to theretention of the solid particles of thermosetting resin, prevents theirmigration within the fibrous material, and leads to the formation of ahighly uniform composite article with no significant variationthroughout its cross section.

Substantially void-free composite articles can be formed from theproduct of the present invention upon the application of heat whichexceeds the melting temperature of the thermosetting resin particles orreaches the temperature at which the thermosetting resin particlesbecome heat-sinterable followed by curing. In preferred embodiments suchtemperature is below that at which any significant crosslinking of theresin occurs during the initial melting and/or heat sintering, and thetemperature subsequently is raised to a higher temperature at whichsubstantial crosslinking takes place. The thermosetting resin becomessubstantially completely cured to form the matrix phase of the resultingcomposite article. The major portion of the aqueous medium may beremoved when the fibrous material is either inside or outside the mold.During composite formation pressure also is applied and a means commonlyis provided for the removal of volatilized aqueous medium and any othergaseous by-products present therein such as those resulting from thecuring reaction. Typical mold pressures during composite formation areapproximately 0.3 to 4 MPa (e.g., 0.6 to 2.5 MPa). The substantiallyvoid-free nature of the product is manifest by less than a two percentvoid content in the composite article which is produced (preferably lessthan one percent void content).

In a preferred embodiment the product of the present invention iscapable of forming a composite article having a zero degree flexuralstrength of at least 60 percent of the theoretical value when tested inaccordance with ASTM D790-84a, Method II, Procedure A, at aspan-to-depth ratio of 32:1, and most preferably at least 70 percent ofthe theoretical value.

Also, in a preferred embodiment the product of the present invention iscapable of forming a composite article having a zero degree flexuralmodulus of at least 80 percent of the theoretical value when tested inaccordance with ASTM D790a, Method II, Procedure A, at a span-to-depthratio of 32:1, and most preferably at least 85 percent of thetheoretical value.

The theoretical flexural modulus value of a fiber-reinforced compositecan be defined as follows: ##EQU2## E_(F) (theoretical)=theoreticalflexural modulus of the composite, E_(T) =tensile modulus of thecomposite,

E_(c) =compressive modulus of the composite,

E_(f) =tensile modulus of the fiber,

E_(m) =flexural modulus of the matrix, and

V_(f) =volume fraction of the fiber.

In order to determine the percent translation of the theoreticallyattainable flexural modulus the following equation is used: ##EQU3##

The theoretical flexural strength value and the percent translation ofthe theoretically attainable flexural strength equations have beensimplified to the following for ease of calculation:

S_(F) (theoretical)=V_(f) S_(f), where

S_(F) (theoretical)=theoretical flexural strength of the composite,

S_(f) =tensile strength of fiber, and

V_(f) =volume fraction of the fiber.

In order to determine the percent translation of the theoreticallyattainable flexural strength the following equation is used: ##EQU4##

The improved composite article mechanical properties made possible whenusing a dissolved polyacrylic acid binding agent possessing across-linked molecular structure and ammonia or an alkyl amine to adjustthe pH are particularly apparent when one examines the 90° tensilestrength values in accordance with ASTM D3039. In a preferredembodiment, the product of the present invention is capable of forming acomposite article having a 90° tensile strength in accordance with ASTMD3039 of at least 60 percent of the theoretical value, and mostpreferably at least 50 percent of the theoretical value.

The theoretical 90° tensile strength value of a fiber-reinforcedcomposite can be defined as follows:

S_(TT) (theoretical)=S_(TM), where

S_(TT) (theoretical=theoretical 90° tensile strength of the composite,and

S_(TM) =tensile strength of the thermosetting resin used in the matrix.

In order to determine the percent translation of the theoreticallyattainable tensile strength the following equation is used: ##EQU5##

The theory whereby the use of ammonia and/or an alkyl amine to form agelled impregnation bath leads to the formation of a composite articlewhich exhibits significantly improved mechanical properties(particularly in the transverse direction) is considered to be complexand incapable of simple explanation. It is believed, however, that theimprovement in transverse properties can be traced to the creation ofenhanced adhesion between the fibers and the matrix formed by thethermosetting resin which is lacking if a metallic base, such as sodiumhydroxide, is utilized.

Reference now is made to the drawings for a discussion of preferredimpregnation apparatus arrangements for use when carrying out theprocess of the present invention. With reference to FIGS. 1 and 3, asupport 102 has affixed thereto supporting members 104 for a pluralityof spools of fibrous tow 106 spaced across a surface thereof. Thefibrous tows 108 while shown as a singular line in reality are comprisedof a plurality of substantially parallel filaments. The tows aremaintained under tension as best shown in FIG. 4 by means of spacedrollers 110, 112 and 114 in a spaced relationship to each other on asupport 116. The tows 108 are then passed through a series of eyelets120 shown in FIG. 5 which are spaced apart from each other on supportmember 122. The plurality of tows is then passed around a series of rodsbest shown in FIG. 6 and identified by reference numerals 124, 126, and128, each of which is supported on member 130. As the tows 108 proceedthrough the aligning apparatus they begin to take on a sheet, fabric, ortape-like unidirectional configuration. The rods 124, 126, and 128 aresupported between members 130 and 130', as shown in FIG. 2. There couldbe a greater number of rods comprising the aligning rods 124, 126, and128 as desired. To further assist in the aligning of the fibrous towsand to form a more uniform sheet-like tape, the plurality of tows arefed to a grooved roller 132 having grooves 134 best shown in FIGS. 7 and10. A roller 138 is eccentrically mounted on shaft 136 and rotates inclockwise direction as best shown in FIGS. 7, 8, and 9 so that the towsmay be alternatingly lifted and uniformly spread about the groovedroller 132. This is best shown comparing the rotation of theeccentrically mounted roller in FIG. 8 to that shown in FIG. 9. Theeccentrically mounted roller 138 has a smooth surface which facilitatesmovement of the tows over the roller. The grooved roller and theeccentrically mounted roller are supported on members 140 and 140,. Aplurality of these eccentric and grooved rollers are shown in aligningsection 144. Positioning roller 133 passes the tows to the eccentricallymounted roller 138.

After the aligning step, the tows are impregnated in the impregnationsection 150 of FIG. 12 where the tows 108 are in a parallel side-by-sidearrangement with each tow 108 abutting another tow. The sheet-like tapeis then passed over a plurality of perforated stainless steel tubes 152.The impregnation bath maintained in a plurality of reservoirs 156 ispumped under a pressure maintained by pressure valves 158 through inlettubes 160 and through the perforations 162 of the tubes 152. Theperforated tubes 152 are maintained in position in section 150 by nuts164 and 164'. The liquid impregnation bath is forced to flow between theadjoining filaments of the sheet-like tape both from above and below asit passes over and under the perforated tubes 152 thereby insuring thatthe fibrous bundles are thoroughly impregnated to the desired level withthe impregnation bath. To insure that the maximum amount of impregnationbath is applied to the moving fibrous tows, the apertures 162 aredistributed only over a radius portion 166 of the tube 152 which is incontact with the sheet-like tape. The fibrous tow passes into theimpregnation section by passing over inlet positioning rod 168 and exitsthe impregnation section by passing over outlet positioning rod 170.

After the sheet-like tape is impregnated at station 150, it is sized tothe desired form by passing through the die section 174 comprised ofupper and lower die members 176 and 178 respectively. These die membershave pointed sections 180 and 182, respectively, to pinch or nip theimpregnated sheet-like tape 184 thereby causing excess impregnation bath186 to remain in the impregnation section 150. By the action of the die,the impregnation bath more thoroughly impregnates the sheet-like tape.The impregnation bath acts as a lubricant as the impregnated sheet-liketape passes through die members 176 and 178. The die members maintain apressure on the sheet-like tape by a tensioning arrangement 190 and190,. The tensioning arrangement, of FIG. 15, permits control of atleast die member 176 to adjust the spacing of pointed members 180 and182. The impregnated tape passes into the die members 176 and 178 whilepassing over trough 192. The impregnation bath 186, in excess of thatrequired for impregnation of the tape, is pinched out by the die membersonto the trough 192 for return to the impregnation section 150. The diemembers are supported in upright members 194, 196. The overall flow ofthe sheet-like tape 108 to form the impregnated tape 184 is best shownin FIG. 2. The dies allow production of materials to a desired aerialweight and dimension (thickness and width).

After the impregnated sheet-like tape leaves the die members, it thenproceeds to a series of rollers which squeeze the impregnated tape inorder to fully spread the impregnation bath in, through and about thefibrous material that makes up the tape as is shown in the bottom leftportion of FIG. 1, and more specifically in FIG. 18. The impregnatedtape passes through a series of opposed rollers 200 and 202 maintainedunder tension by a tensioning device 204 and 204'. The tensioning deviceis arranged in upright members 206 and 206' and utilizes a threaded boltmember 208 to keep the upper roller 200 tightly against the impregnatedtape 184 as the tape moves through the opposed rollers. The rollers arelocated in roller section 210.

After the impregnated sheet-like tape passes through roller section 210,a fine coating spray of the impregnation bath optionally is applied inthe spray chamber 212. The spray chamber may or may not be useddepending upon the processing characteristics desired in the impregnatedsheet-like tape. In a preferred embodiment no coating is applied.

Thereafter, the impregnated tape may be dried to remove a portion of theaqueous medium of the impregnation bath in heating chamber 214 bypassing the impregnated sheet-like tape between heating elements. InFIG. 19 is shown upper and lower belts 216 and 218, respectively whichare continuous belts moving in the same direction and which may beperforated polymeric belts, such as Teflon polytetrafluoroethylenebelts. Preferably the heating chamber maintains the temperature at anappropriate level to evaporate in a controlled manner a portion of theaqueous or volatile materials present in the impregnated sheet-liketape. It is to be appreciated that the appropriate concentration ofaqueous medium in the sheet-like tape may be controlled with precisionin the heating chamber. The moving belts are held in position above andbelow the impregnated tape 184 by use of a plurality of bars, supportsor rollers 220. Electric resistance heating elements (or steam heatingelements) 222 control the drying temperature in the drying unit 214. Itis to be appreciated that the moisture content of the impregnated tapecan vary depending on the drying time and temperature.

If desired, an effective amount of an adhesive may be sprayed in spraychamber 224 as the dried impregnated tape passes. However, in apreferred embodiment no adhesive is applied.

Subsequent to the spray chamber 224, the impregnated sheet-like tape 184is formed into a roll 226 supported on shaft 226A. During the rollforming process, the tape is covered along top and bottom sides byprotective sheet material 228. As shown in FIG. 20, a support member 231has mounted thereon a pair of shafts 233, each of which carries a rollof the protective sheet material 229. As the tape 184 is fed to theshaft 226A, the protective sheet material in the form of webs 228 coversthe top and bottom sides of the tape. The triple-ply arrangement formsthe roll 226.

The impregnation of a performed woven or knitted fabric comprising endsof a plurality of adjoining substantially parallel filaments isillustrated in FIG. 21. Supply roll 230 feeds fabric 232 to immersiontank 234 containing the impregnation bath as described herein. Thefabric 232 passes over and under a plurality of perforated rollers 236which are comparable to rollers 152 as best shown in FIGS. 11 and 12.After the fabric is impregnated, it passes through heating zones 242having opposed belts 240 with supporting bars 238 and heating elements244 comparable to that shown in FIGS. 19.

While FIG. 21 shows separate immersion tanks, it is to be appreciatedthat any number may be utilized as desired to appropriately impregnatethe fabric. Different concentrations of particles of thermosetting resinmay be present in the tanks to obtain the desired impregnation results.The concentration may be up to 15 percent (e.g., 10 to 15 percent) byweight of the particles of thermosetting resin in step 1, up to about 25percent (e.g., 20 to 25 percent) by weight of the particles ofthermosetting resin in step 2, and up to 35 percent (e.g., 30 to 35percent) by weight of particles of thermosetting resin in step 3. Theimpregnated fabric leaving the final drier could then be taken up on aroller as desired.

Regardless of the technique used for preparing the impregnated fibrousmaterial, a laminate may be prepared from the same and shaped into adesirable configuration. Because the present invention obtains a tackyand drapable product that may be shaped to a predetermined configurationin a relatively easy manner, composite articles having a complexconfiguration readily can be formed. For instance, a plurality ofimpregnated sheet-like tapes 184 such as those obtained from theapparatus as best shown in FIG. 1, can be cut to the desired size. Thesheets are then placed into an opposed pair of mold members 250 and 252as best shown in FIGS. 22 and 23. By the application of appropriate heatand pressure the final configured part 254 is obtained. The applicationof known mold release coatings such as silicon-based materials aregenerally applied prior to the insertion into mold members 250 and 252.

The final shaped part 254 is a composite laminate having usefulmechanical properties. It is characterized as being lightweight, and maybe used in the formation of various aerospace and automotive components.

An alternative or a conjunctive piece of apparatus to impregnationsection 150 is immersion chamber 260 with a reservoir of theimpregnation bath 262 retained therein. FIG. 13 shows a series of movingrollers 264 which the fibrous sheet-like tape passes over and under.While it may be somewhat duplicative, under appropriate circumstances,one may need additional impregnation into the sheet-like tape.Accordingly, a perforated tube 152 could be used in place of one or allof the rollers 264 as shown in immersion chamber 260. An alternativeembodiment to the use of the perforated roller 152 would be to spray thefibrous tow in spray chamber 270 as shown in FIG. 14. Spray members 272and 274 could be utilized to spray the impregnation bath above and belowthe fibrous sheet-like tape as it moves through the chamber. Againrollers 276 could be configured to be perforated such as perforatedroller 152 shown in FIG. 11.

A further apparatus to impregnate to fibrous material is illustrated inFIGS. 25 to 29. FIG. 25 is a schematic diagram of an overall apparatusto be utilized in the present invention. A plurality of tows 312 are fedto the impregnating apparatus 310 from spools 314. A particularlypreferred tow would contain 12,000 substantially parallel carbonfilaments per tow of Celion carbon fibers available from BASF StructuralMaterials, Inc., Charlotte, N.C., U.S.A. Approximately 60 spools can beprovided at position 314. The tow is maintained under tension betweenrollers 316, 318 and introductory roller 320. A heating chamber 322volatilizes any sizing that may be present on the tow. A temperaturerange in the heating chamber 322 may be from 400° C. to 925° C.,preferably about 600° C. In addition to or as an alternative to heatingchamber 322, the tow can be passed over roller 326 and through a solventreservoir 324. The solvent therein could be used to remove any sizingagent that may be present. Suitable solvents would be the aromatic,chlorinated aliphatic and heterocyclic materials such as methylethylketone, methylene chloride, acetone, xylene, methylpyrrolidone. Rollers330 underneath the level of solvent 328 may be adjusted to provide anappropriate residence time in the solvent tank.

After the fibrous material leaves the solvent tank, it is thenimpregnated with the impregnation bath. A number of different techniquescan be utilized to accomplish the desired impregnation through theapplication of work. The only requirement is that the impregnation bathwell flow between adjoining filaments. The objective is to have asminimal voids as possible, i.e., minimal portions of the tow that arenot in contact with particulate thermosetting resin.

FIG. 25 shows the tow 312 being immersed from top to bottom in animmersion tank of chamber 332 which includes heating elements 334 and334' for bath temperature control positioned in heat element chambers335 and having respective wire leads 33 and 336'. The fibrous material312 as it proceeds through the impregnation bath 338 is wound aboutmultiple rollers 340 and 342. Roller 342 slides in slot 344 back andforth to permit appropriate tension on the fibrous material. Also, theroller 342 may be replaced by a bar arrangement which would permit thefilaments of the fibrous material to be spread out on the bar, therebypermitting the thermosetting resin particles to further surround thefilaments. Also, there may be multiple bars that can take the place ofroll 342 variously placed in the immersion tank of chamber 332. Further,utilization of ultrasonic equipment attached to either the rollersand/or the bars would permit further surrounding of the tow with theimpregnation bath thereby improving the impregnation. The ultrasonicequipment causes movement of the fibers and the particles ofthermosetting resin thereby increasing the impregnation of the fibrousmaterial. The ultrasonic equipment is readily available in the industry,such as Sonicator ultrasonic equipment, commercially available from HeatSystems, Inc. of Farmingdale, N.Y., U.S.A.

When utilizing the apparatus shown in FIG. 25, the fibrous materialpasses from the immersion tank of chamber 332 through a die (shown inFIGS. 28 and 29) which will size the fibrous material. Shown in FIG. 25is a heating chamber 346 which can control the temperature of the tow asit leaves the heated impregnation tank or chamber. In this case one maybe able to dry the impregnated fibrous material to a particular aqueousmedium content, in a controlled manner. The resulting fibrous materialis then ready for take-up on take-up roll 360 after passing overintroductory roller 358.

The tows impregnated through the use of the equipment heretoforedescribed instead of being a collection of individual filaments take onthe shape of a tape or sheet. Such tape or sheet can vary in size. Evenif a narrow tape is prepared, the tape adheres well on a roller due tothe tacky nature of the resulting impregnated fibrous material to anadjacent wrap which is laid next to it in a side-by-side relationship.The dissolved polymeric binding agent (previously described) assists inadhering the thermosetting resin particles to the filaments as well aswithin tows which comprise the tape. Due to the complete impregnation ofthe fibrous material with the impregnation bath, the resulting productis "drapable", that is, it has the ability to hang or stretch outloosely and is capable of being readily easily folded. By having thisdrapable capability, the multiple tows now in the sheet-like form can beshaped to any desired configuration, such as in a die for forming a bulkhead door of an aircraft. Once in the die, it can be molded at anappropriate temperature and pressure. Other aircraft end uses could belanding gear doors, cowl components, wing to body fairing, outboardailerons, stabilizer tips, rudders, elevator wings, fuselage, and thelike.

Since the apparatus that can be utilized to impregnate the fibrousmaterial shown in FIG. 25 is relatively light, it can be affixed to anupright member 362 which is connected to a supporting stand 364.

Since some of the thermosetting resins that may be useful in the presentprocess are expensive, it has been found to be desirable to retain theimpregnation bath that is applied to the tow in an immersion tank orchamber that has multiple chambers such as that shown in FIG. 26, whichis a top view of the chamber 332. Shown on the left side of theapparatus is the chamber 370 in which the impregnation bath 338 isretained. The chamber comprises a top member 372 and a bottom member 374which are bolted together by threaded fastener means 376 and 378 withthe face means 380, best shown in FIG. 27, having the sides 382 securedby similar fastening members 384 and 386. Basically, the operation ofthe multi-chambered immersion tank or chamber permits the impregnationbath used to impregnate the fibrous material, to be pumped through pump390 through inlet 392 into a first reservoir 394. A piston having a head396 is pressed down through the chamber 394 by a handle or rod 398. Aconduit 400 connects the reservoir 394 with reservoir chamber 402. Thehandle 398 is biased open by spring 405. The material that is present inchamber 402 can be passed into the contact chamber 370 by means ofpiston rod 404 which acts similar to piston rod 398 in the firstreservoir chamber. In this fashion, the impregnation bath is maintainedat an appropriate level by means of weir 403 which serves to connect anddisconnect the chambers 394 and 402. Shaping die 410 is best shown inFIG. 28 which is comprised of aligned aperture 412 in cooperatingelements 410 and 410'.

To facilitate the appropriate tension within the chamber 332 the rollers340 may have cooperating bars or rollers 414 placed on a manifold 416which can be adjusted to a desired tension by movement of bar member 418which pivots at point 417. Use of ultrasonic techniques can likewise beapplied to manifold 416 as could be applied to the bar that could takethe place of roller 342 as discussed previously. Bar 418 would in turnbe attached to servomotor 420 for frequent movement of the bar andmanifold about pivot 417 as desired. Weir 424 operates in a similarfashion to weir 403 and are movable in and out of position by means ofhandles 426 and 428, respectively. The weirs can be slid in and out ofposition to permit the material to flow from one chamber to the otherand into the immersion tank or chamber 370. The rod 404 is biased openby spring 405'.

The following examples are presented as specific illustrations of theclaimed invention. It should be understood, however, that the inventionis not limited to the specific details set forth in the examples.

EXAMPLE I

An alkyl phenoxypolyethoxy ethanol surfactant in a quantity of 2.5 gramsis dissolved in 3362 grams of distilled water and combined at ambienttemperature conditions with an aqueous dispersion of 750 grams of solidparticles of M-100 PMR15 polyimide thermosetting resin in 780 grams ofwater. The M-100 PMR-15 polyimide thermosetting resin is of the reverseDiels-Alder type and is commercially available from Hysol Grafil,Cleveland, Ohio, U.S.A., and is fully imidized as purchased. Once thispolyimide resin is postcured via an addition crosslinking reaction itsglass transition temperature is typically approximately 321° C. Theparticles of M-100 PMR-15 polyimide thermosetting resin have been groundto a mean particle size of 4.8 microns, with the largest particle sizebeing approximately 24 microns. The surfactant is commercially availablefrom Rohm and Haas Company as Triton X100 surfactant.

38 grams of solid particles of water-soluble polyacrylic acid bindingagent possessing a molecular structure which is cross-linked withpolyalkenyl polyether are slowly added to an agitated bath of theaqueous powder dispersion mentioned above. The water-soluble bindingagent is commercially available from B.F. Goodrich as Grade 941 Carbopolpolymer and has a molecular weight of approximately 1,250,000.

Mixing is conducted for two hours in order to completely dissolve thewater-soluble binding agent and to disperse the particles of M-100PMR-15 polyimide thermosetting resin. The pH of the resulting dispersionis found to be 3.2. The viscosity of the dispersion is found to be160,000 cps. when measured with a Rheometrics Stress Rheometer (ModelRSR/M) while operating at a shear rate of 0.01 reciprocal second. Thisviscosity is accompanied by a high shear-thinning behavior which enablesgood dispersion of the particles of thermosetting resin while beingagitated. The Brookfield Yield Value of the dispersion is found to be 70dynes/cm.² when tested on a Brookfield RVT viscometer as previouslydescribed.

70 grams of a 9.3 percent ammonium hydroxide solution are then added tothe moderately agitated dispersion. This raises the pH of the dispersionto 7.5 to form an improved gelled impregnation bath wherein theviscosity of the resulting bath is substantially increased toapproximately 1,200,000 cps. as measured with a Rheometrics StressRheometer (Model RSR/M) at a shear rate of 0.01 reciprocal secondthrough an extension of the molecules of the dissolved binding agent.The resulting impregnation bath exhibits plastic flow withshear-thinning behavior. The Brookfield Yield Value of the dispersion isfound to be 360 dynes/cm.² when tested on a Brookfield RVT viscometer aspreviously described. It can be calculated as previously described thatthe Minimum Brookfield Yield Value required to suspend the largestparticles of M-100 PMR-15 thermosetting polyimide resin present in thedispersion is 4.1 dynes/cm.² Accordingly, the resulting impregnationbath is highly stable and the actual Brookfield Yield Value exceeds thecalculated Minimum Brookfield Value to suspend even the largest M-100PMR-15 polyimide thermosetting resin particles present by more than 85times.

The resulting impregnation bath is next poured into an impregnationapparatus similar to that illustrated in FIG. 25 containing severalstationary non-rotating bars immersed within the bath.

A tow of approximately 12,000 substantially continuous Celion carbonfilaments in unsized form available from BASF Structural Materials,Inc., under the designation G30-500 is selected to serve as the fibrousreinforcement. The filaments of the tow possess a diameter ofapproximately 7 microns. This tow is fed in the direction of its lengthfrom a single bobbin located on a tension controlled creel, and whileunder tension, is passed through the impregnation bath while in contactwith the stationary non-rotating bars immersed in the bath. Followingpassage over the bars the multifilamentary tow is passed through arectangular metallic die having polished surfaces situated at the bottomof the impregnation bath. While passing over the bars and through thedie, the substantially parallel carbon filaments are impregnated withthe impregnation bath as the bath is caused to flow between theadjoining filaments. Such flow inherently results in a significantreduction in the viscosity of the impregnation bath which greatly aidsin the incorporation of the solid M-100 PMR-15 polyimide thermosettingresin particles between the carbon filaments. Once the flow isdiscontinued the M-100 PMR-15 thermosetting resin particles tend to belocked within the fibrous material in a highly uniform manner. The diealso aids in the control of the width and thickness of the resultingimpregnated tow.

The resulting impregnated tow is next wound on a rotating drum using atransversing laydown guide to form a rectangular sheet having a width ofapproximately 30.5 cm.

It is found that the resulting sheet product following drying for twohours at ambient conditions to remove a portion of the water containsapproximately 30.1 percent carbon fibers by weight, approximately 13.2percent solid M-100 PMR-15 polyimide thermosetting resin particles byweight, approximately 0.7 percent ammonium polyacrylate binding agenthaving stiffened molecules possessing a cross-linked molecular structureby weight, and approximately 56 percent water by weight. This productcontains the matrix-forming M-100 PMR-15 thermosetting resin particlessubstantially uniformly dispersed between adjoining fibers in theabsence of fusion bonding and can be handled without the segregation ofthe particles. When tested in accordance with the drape test of ASTMD-1388, the product is found to be highly drapeable both in the 0° and90° directions and to exhibit a flexural rigidity of approximately10,000 mg..cm. Also, the product is tacky and is found to pass the tacktest of NASA Technical Bulletin 1142.

Next, the product while in a flat configuration is allowed tosubstantially completely dry to a water content of approximately 2percent while at ambient conditions, and contains the M-100 PMR-15thermosetting resin particles well bound therein. Ten flat pliescontaining 31.6 percent by weight of the M-100 PMR-15 thermosettingresin particles measuring approximately 10.2 cm.×10.2 cm. are laid up inthe 0° direction in a matched metal mold at room temperature conditionsand placed in a platen press. The mold is heated to 275° C. with noapplied pressure. Once this temperature is achieved, a pressure of 2.1MPa is applied. The mold is then held at these conditions forapproximately 30 minutes, after which the mold is heated up to 300° C.and held for another 30 minutes. The thermosetting resin issubstantially completely cured to form the matrix phase. The press isthen turned off, thereby allowing the mold to slowly cool under 2.1 MPapressure. Once the mold cools to 50° C., it is removed from the pressand the resulting composite article in the form of a panel is removedfrom the mold. The resulting panel is postcured for 13 hours at 315° C.in a circulating air oven. It is then allowed to cool slowly to ambienttemperature.

Alternatively, it would be possible to lay up the impregnated fibrousmaterial prior to drying while still in the wet, tacky form. Under suchconditions the open mold, containing the plies, could initially beheated to approximately 120° to 150° C. for approximately 1 hour in theabsence of pressure, to further assist in the removal of volatiles. Suchprocedure would be particularly advantageous when forming a compositearticle of a more complex configuration, wherein drape and tack are ofgreater importance.

The resulting panel formed from the impregnated product, which is driedprior to placing in the mold, has a thickness of 0.154±0.001 cm., atheoretical carbon fiber volume of 61.2 percent, and a void content ofless than 1.5 percent.

Test specimens are cut from panels, molded in the above described mannerand four point, 0° flexural tests are conducted in accordance with ASTMD790-84a, Method II, Procedure A, using a span-to-depth ratio ofapproximately 32:1 and a crosshead speed of approximately 0.5cm./minute, and 90° tensile tests are conducted in accordance with theprocedure of ASTM D3039. A 0° flexural strength of 1550 MPa isexhibited, which represents a 66 percent translation of thattheoretically attainable. A 0° flexural modulus of 122 GPa is exhibited,which represents an 86 percent translation of that theoreticallyattainable. A 90° tensile strength of 40 MPa is exhibited whichrepresents a 72 percent translation of that theoretically attainable.Such 90° tensile strength value evidences superior adhesion between thecarbon fibers and the matrix formed upon the substantially completecuring of the thermosetting resin.

EXAMPLE II

An alkyl phenoxypolyethoxy ethanol surfactant in a quantity of 0.75grams is dissolved in 1230 grams of distilled water and combined atambient temperature conditions with 223 grams of solid particles of amodified 5250-2 poly(bismaleimide) thermosetting resin formed inaccordance with the teachings of U.S. Pat. No. 4,644,039. The modified5250-2 poly(bismaleimide) thermosetting resin is supplied by BASFAktiengesellschaft of Ludwigshafen, West Germany and is advanced toproduce a melting temperature of approximately 70° to 80° C. and a geltime of approximately 10 minutes at 160° C. Since the thermosettingresin is a solid at room temperature, it can be ground to powder form.Once this modified 5250-2 poly(bismaleimide) thermosetting resin ispostcured, its glass transition temperature is typically approximately321° C. The particles of poly(bismaleimide) thermosetting resin havebeen ground to a mean particle size of 25 microns, with the largestparticle size being 130 microns. The surfactant is commerciallyavailable from Rohm and Haas Company as Triton X100 surfactant.

10 grams of solid particles of water-soluble polyacrylic acid bindingagent possessing a molecular structure which is cross-linked withpolyalkenyl polyether are slowly added to an agitated bath of theaqueous powder dispersion mentioned above. The water-soluble bindingagent is commercially available from B.F. Goodrich as Grade 941 Carbopolresin and has a molecular weight of approximately 1,250,000.

Mixing is conducted for two hours in order to completely dissolve thewater-soluble binding agent and to disperse the particles ofpoly(bismaleimide) thermosetting resin. The pH of the resultingdispersion is found to be 3.7. The viscosity of the dispersion is foundto be 120,000 cps. when measured with a Rheometrics Stress Rheometer(Model RSR/M) while operating at a shear rate of 0.01 reciprocal second.This viscosity is accompanied by a high shear-thinning behavior whichenables good dispersion of the particles of thermosetting resin. TheBrookfield Yield Value of the dispersion is found to be 45 dynes/cm.²when tested on a Brookfield RVT viscometer as previously described.

20 grams of a 9.3 percent ammonium hydroxide solution are then added tothe moderately agitated dispersion. This raises the pH of the dispersionto 7.0 to form an improved gelled impregnation bath wherein theviscosity of the resulting bath is substantially increased toapproximately 900,000 cps. when measured with a Rheometrics StressRheometer (Model RSR/M) at a shear rate of 0.01 reciprocal secondthrough an extension of the molecules of the dissolved binding agent.The resulting impregnation bath exhibits plastic flow withshear-thinning behavior. The Brookfield Yield Value of the dispersion isfound to be 280 dynes/cm.² when tested on a Brookfield RVT viscometer aspreviously described. It can be calculated as previously described thatthe Minimum Brookfield Yield Value required to suspend the largestparticles of the poly(bismaleimide) thermosetting resin present in thedispersion is 10.9 dynes/cm.². Accordingly, the resulting impregnationbath is highly stable and the actual Brookfield Yield Value exceeds thecalculated Minimum Brookfield Value to suspend even the largestpoly(bismaleimide) thermosetting resin particles present by more than 25times.

The resulting impregnation bath is next poured into an impregnationapparatus similar to that illustrated in FIG. 25 containing severalstationary non-rotating bars immersed within the bath.

A tow of approximately 12,000 substantially continuous Celion carbonfilaments in unsized form available from BASF Structural Materials,Inc., under the designation G30-500 is selected to serve as the fibrousreinforcement. The filaments of the tow possess a diameter ofapproximately 7 microns. This tow is fed in the direction of its lengthfrom a single bobbin located on a tension controlled creel, and whileunder tension, is passed through the impregnation bath while in contactwith the stationary non-rotating bars immersed in the bath. Followingpassage over the bars the multifilamentary tow is passed through arectangular metallic die having polished surfaces situated at the bottomof the impregnation bath. While passing over the bars and through thedie the substantially parallel carbon filaments are impregnated with theimpregnation bath as the bath is caused to flow between the adjoiningfilaments. Such flow inherently results in a significant reduction inthe viscosity of the impregnation bath which greatly aids in theincorporation of the solid poly(bismaleimide) thermosetting resinparticles between the carbon filaments. Once the flow is discontinuedthe poly(bismaleimide) thermosetting resin particles tend to be lockedwithin the fibrous material in a highly uniform manner. The die alsoaids in the control of the width and thickness of the resultingimpregnated tow.

The resulting impregnated tow is next wound on a rotating drum using atransversing laydown guide to form a rectangular sheet having a width ofapproximately 30.5 cm.

It is found that the resulting sheet product following drying for twohours at ambient conditions to remove a portion of the water containsapproximately 29.7 percent carbon fibers by weight, approximately 14.6percent solid poly(bismaleimide) thermosetting resin particles byweight, approximately 0.7 percent ammonium polyacrylate binding agenthaving stiffened molecules possessing a cross-linked molecular structureby weight, and approximately 55 percent water by weight. This productcontains the matrix-forming poly(bismaleimide) thermosetting resinparticles substantially uniformly dispersed between adjoining fibers inthe absence of fusion bonding and can be handled without the segregationof the particles. When tested in accordance with the drape test of ASTMD-1388 the product is found to be highly drapeable both in the 0° and90° directions and to exhibit a flexural rigidity of approximately10,000 mg..cm. Also, the product is tacky and is found to pass the tacktest of NASA Technical Bulletin 1142.

Next, the product while in a flat configuration is allowed tosubstantially completely dry to a water content of approximately 2percent while at ambient conditions, and contains the poly(bismaleimide)thermosetting resin particles well bound therein. Ten flat pliescontaining 34.0 percent by weight of the poly(bismaleimide)thermosetting resin particles, measuring approximately 10.2 cm.×10.2 cm.are laid up in the 0° direction in a matched metal mold at roomtemperature conditions and placed in a platen press. The mold is heatedto 150° C. with no applied pressure, at a rate of 1.5° C./minute. Oncethis temperature is achieved, a pressure of 1.4 MPa is applied. The moldis then held at these conditions for approximately 60 minutes, afterwhich the mold is heated up to 180° C. and held for another 6 hours. Thethermosetting resin is substantially completely cured to form the matrixphase. The press is then turned off thereby allowing the mold to slowlycool under 1.4 MPa pressure. Once the mold cools to 50° C. it is removedfrom the press and the resulting composite article in the form of apanel is removed from the mold. The resulting panel is postcured for 12hours at 230° C. in a circulating air oven. It is then allowed to coolslowly to ambient temperature.

Alternatively, it would be possible to lay up the impregnated fibrousmaterial prior to drying while still in the wet, tacky form. Under suchconditions the open mold, containing the plies, could initially beheated to approximately 40° to 50° C. for approximately 2 hours in avacuum oven to further assist in the removal of volatiles. Suchprocedure would be particularly advantageous when forming a compositearticle of a more complex configuration, wherein drape and tack are ofgreater importance.

The resulting panel formed from the impregnated product, which is driedprior to placing in the mold, has a thickness of 0.180±0.001 cm., atheoretical carbon fiber volume of 57.5 percent, and a void content ofless than 1.5 percent.

Test specimens are cut from panels, molded in the above describedmanner, and four point, 0° flexural tests are conducted in accordancewith ASTM D790-84a, Method II, Procedure A, using a span-to-depth ratioof approximately 32:1 and a crosshead speed of approximately 0.5cm./minute, and 90° tensile tests are conducted in accordance with ASTMD3039. A 0° flexural strength of 1500 MPa is exhibited, which representsa 67 percent translation of that theoretically attainable. A 0° flexuralmodulus of 122 GPa is exhibited, which represents a 92 percenttranslation of that theoretically attainable. A 90° tensile strength of41 MPa is exhibited which represents a 65 percent translation of thattheoretically attainable. Such 90° tensile strength value evidencessuperior adhesion between the carbon fibers and the matrix formed uponthe substantially complete curing of the thermosetting resin.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be employedwithout departing from the concept of the invention as defined in thefollowing claims.

We claim:
 1. A method for the production of an improved fibrous materialsuitable for the formation of a substantially void-free compositearticle comprising a plurality of adjoining substantially parallelreinforcing filaments in association with a matrix-forming thermosettingresin comprising:(a) preparing a dispersion of solid particles of athermosetting resin in an aqueous medium which contains an effectiveamount of a dissolved polymeric binding agent, (b) substantiallyincreasing the viscosity of said dispersion to form an improvedimpregnation bath whereby the viscosity of the resulting bath becomes atleast 50,000 cps. and said impregnation bath has a plastic flowcharacteristic with shear-thinning behavior which is sufficient tosubstantially uniformly suspend said particulate thermosetting resinwithin said bath, (c) impregnating said adjoining substantially parallelreinforcing filaments with said bath under conditions wherein said bathis caused to flow between said adjoining filaments by the application ofwork wherein said bath flow inherently results in a reduction of theviscosity of said bath which aids in the incorporation of saidparticulate thermosetting resin between adjoining filaments, and (d)controlling the content of said aqueous medium in the resulting fibrousmaterial to provide a product having said particles of matrix-formingthermosetting resin substantially uniformly dispersed between adjoiningfilaments in the absence of fusion bonding which inherently (1) isdrapable and tacky at ambient conditions, (2) is handleable withoutsegregation of said particles within the fibrous material, and (3) whichupon the application of heat and pressure can be transformed into asubstantially void-free fiber-reinforced composite article of apredetermined configuration wherein said thermosetting resin becomessubstantially completely cured and forms the matrix phase.
 2. A methodin accordance with to claim 1 wherein said plurality of adjoiningsubstantially parallel reinforcing filaments are provided as a singleend.
 3. A method in accordance with claim 1 wherein said fibrousmaterial comprises a plurality of ends each comprising a plurality ofsubstantially parallel reinforcing filaments.
 4. A method in accordancewith claim 1 wherein said fibrous material is in the configuration of acloth which incorporates a plurality of ends each comprising a pluralityof substantially parallel reinforcing filaments.
 5. A method inaccordance with claim 1 wherein said reinforcing filaments are selectedfrom the group consisting of carbon, glass, aramid, silicon carbide,silicon nitride, boron nitride, and mixtures of the foregoing.
 6. Amethod in accordance with claim 1 wherein said reinforcing filaments arecarbon filaments.
 7. A method in accordance with claim 1 wherein saidreinforcing filaments are glass filaments.
 8. A method in accordancewith claim 1 wherein said solid particles of thermosetting resin areselected from the group consisting of phenolic resins, polyester resins,melamine-formaldehyde resins, urea-formaldehyde resins,casein-formaldehyde resins, polyimide resins, polyurethane resins, epoxyresins, diallyl phthalate resins, vinyl ester resins, polybutadiene(1,2)resins, cyanate ester resins, and cyanamide resins.
 9. A method inaccordance with claim 1 wherein said solid particles of thermosettingresin are polyimide resin.
 10. A method in accordance with claim 9wherein said solid particles of thermosetting resin are a reverseDiels-Alder polyimide resin which is substantially fully imidized and iscapable of undergoing an addition cross-linking reaction in thesubstantial absence of the generation of volatile by-products.
 11. Amethod in accordance with claim 9 wherein said particles ofthermosetting resin are poly(bismaleimide) resin.
 12. A method inaccordance with claim 1 wherein said solid particles of thermosettingresin possess a particle size within the range of approximately 0.1 to100 microns.
 13. A method in accordance with claim 1 wherein said solidparticles of said thermosetting resin are provided in said dispersion ofstep (a) in a concentration of approximately 5 to 50 percent by weightbased upon the total weight of the dispersion.
 14. A method inaccordance with claim 1 wherein said dissolved polymeric binding agentis provided in step (a) in a concentration of approximately 0.01 to 5percent by weight based upon the total weight of the dispersion.
 15. Amethod in accordance with claim 1 wherein the viscosity of saiddispersion is increased in step (b) through the addition of an agentwhich interacts with said dissolved polymeric binding agent.
 16. Amethod in accordance with claim 15 wherein the viscosity of saiddispersion is increased in step (b) through the addition of an agentwhich adjusts the pH.
 17. A method in accordance with claim 1 whereinsaid dispersion provided in step (a) additionally includes a surfactantin a minor concentration to aid in the wetting of said particles ofthermosetting resin.
 18. A method in accordance with claim 1 whereinsaid viscosity is raised at least 50 percent in step (b) to at least50,000 cps.
 19. A method in accordance with claim 1 wherein saidviscosity is raised in step (b) to between approximately 50,000 to3,000,000 cps.
 20. A method in accordance with claim 1 wherein saidviscosity is raised in step (b) to between approximately 50,000 to250,000 cps.
 21. A method in accordance with claim 1 wherein saidimpregnation bath formed in step (b) possesses a Brookfield Yield Valueabove the minimum value required to permanently suspend the largestparticles of said thermosetting resin present in said bath.
 22. A methodin accordance with claim 1 wherein said impregnation of step (c) iscarried out while said reinforcing filaments are immersed in said bathand said work is applied as said filaments while under tension arepassed in contact with at least one solid member.
 23. A method inaccordance with claim 1 wherein said impregnation step (c) is carriedout by passing said substantially parallel reinforcing filaments incontact with the outer surface of at least one perforated tube throughwhich said bath is forced.
 24. A method in accordance with claim 1wherein in step (d) said concentration of aqueous medium in saidresulting fibrous material is controlled within the range ofapproximately 10 to 70 percent by weight.
 25. A method in accordancewith claim 1 wherein in step (d) said resulting fibrous material isdried to remove a portion of the aqueous medium.
 26. A method inaccordance with claim 1 wherein in step (d) said resulting fibrousmaterial is dried to remove at least a portion of the aqueous medium andadditional aqueous medium subsequently is applied thereto in order tomaintain the recited characteristics.
 27. A method in accordance withclaim 1 wherein the product of step (d) contains said particles ofmatrix-forming thermosetting resin in a concentration of approximately 6to 45 percent by weight.
 28. A method for the production of an improvedfibrous material suitable for the formation of a substantially void-freecomposite article comprising a plurality of adjoining substantiallyparallel reinforcing filaments in association with a matrix-formingthermosetting resin comprising:(a) preparing a dispersion of solidparticles of a thermosetting resin in an aqueous medium which containsan effective amount of dissolved polyacrylic acid binding agentpossessing a cross-linked molecular structure, (b) raising the pH ofsaid aqueous medium to form an improved impregnation bath wherein theviscosity of the resulting bath is substantially increased to at least50,000 cps. through stiffening of the molecules of said binding agentand said impregnation bath has a plastic flow characteristic withshear-thinning behavior which is sufficient to substantially uniformlysuspend said particulate thermosetting resin within said bath, (c)impregnating said adjoining substantially parallel reinforcing filamentswith said bath under conditions wherein said bath is caused to flowbetween said adjoining filaments by the application of work wherein saidflow inherently results in a reduction of the viscosity of said bathwhich aids in the incorporation of said particulate thermosetting resinbetween adjoining filaments, and (d) controlling the content of saidaqueous medium in the resulting fibrous material to provide a producthaving said particles of matrix-forming thermosetting resinsubstantially uniformly dispersed between adjoining filaments in theabsence of fusion bonding which inherently (1) is drapable and tacky atambient conditions, (2) is handleable without segregation of saidparticles within the fibrous material, and (3) which upon theapplication of heat and pressure can be transformed into a substantiallyvoid-free fiber-reinforced composite article of a predeterminedconfiguration wherein said thermosetting resin becomes substantiallycompletely cured and forms the matrix phase.
 29. A method in accordancewith claim 28 wherein said plurality of adjoining substantially parallelreinforcing filaments are provided as a single end.
 30. A method inaccordance with claim 28 wherein said fibrous material comprises aplurality of ends each comprising a plurality of substantially parallelreinforcing filaments.
 31. A method in accordance with claim 28 whereinsaid fibrous material is in the configuration of a cloth whichincorporates a plurality of ends each comprising a plurality ofsubstantially parallel reinforcing filaments.
 32. A method in accordancewith claim 28 wherein said reinforcing filaments are selected from thegroup consisting of carbon, glass, aramid, silicon carbide, siliconnitride, boron nitride, and mixtures of the foregoing.
 33. A method inaccordance with claim 28 wherein said reinforcing filaments are carbonfilaments.
 34. A method in accordance with claim 28 wherein saidreinforcing filaments are glass filaments.
 35. A method in accordancewith claim 28 wherein said solid particles of thermosetting resin areselected from the group consisting of phenolic resins, polyester resins,melamine-formaldehyde resins, urea-formaldehyde resins,casein-formaldehyde resins, polyimide resins, polyurethane resins, epoxyresins, diallyl phthalate resins, vinyl ester resins, polybutadiene(1,2)resins, cyanate ester resins, and cyanamide resins.
 36. A method inaccordance with claim 28 wherein said solid particles of thermosettingresin are polyimide resin.
 37. A method in accordance with claim 36wherein said solid particles of thermosetting resin are a reverseDiels-Alder polyimide resin which is substantially fully imidized and iscapable of undergoing an addition cross-linking reaction in thesubstantial absence of the generation of volatile by-products.
 38. Amethod in accordance with claim 36 wherein said particles ofthermosetting resin are poly(bismaleimide) resin.
 39. A method inaccordance with claim 28 wherein said solid particles of saidthermosetting resin possess a particle size within the range ofapproximately 0.1 to 100 microns.
 40. A method in accordance with claim28 wherein said solid particles of said thermosetting resin are providedin said dispersion of step (a) in a concentration of approximately 5 to50 percent by weight based upon the total weight of the dispersion. 41.A method in accordance with claim 28 wherein said dissolved polyacrylicacid binding agent possessing a cross-linked molecular structure has amolecular weight of approximately 450,000 to 4,000,000.
 42. A method inaccordance with claim 28 wherein said dissolved polyacrylic acid bindingagent possessing a cross-linked molecular structure is provided in step(a) in a concentration of approximately 0.01 to 2 percent by weightbased upon the total weight of the dispersion.
 43. A method inaccordance with claim 28 wherein said dissolved polyacrylic acid bindingagent is cross-linked with polyalkenyl polyether.
 44. A method inaccordance with claim 28 wherein said dispersion provided in step (a)additionally includes a surfactant in a minor concentration to aid inthe wetting of said particles of thermosetting resin.
 45. A method inaccordance with claim 28 wherein said dispersion of step (a) possesses apH of approximately 2.5 to 3.5.
 46. A method in accordance with claim 28wherein the viscosity of said dispersion is increased in step (b)through the addition of a base which is dissolved in an aqueous solvent.47. A method in accordance with claim 28 wherein the viscosity of saiddispersion is increased in step (b) through the addition of a baseselected from the group consisting of ammonia, an alkyl amine having aboiling point less than 100° C., and mixtures of the foregoing.
 48. Amethod in accordance with claim 28 wherein the viscosity of saiddispersion is increased in step (b) through the addition of an aqueoussolution of ammonia.
 49. A method in accordance with claim 28 whereinthe viscosity of said dispersion is increased in step (b) through theaddition of an aqueous solution of a base selected from the groupconsisting of methylamine, ethylamine, triethylamine, and mixtures ofthe foregoing.
 50. A method in accordance with claim 28 wherein in step(b) the pH is raised to approximately 4 to
 10. 51. A method inaccordance with claim 28 wherein said viscosity is raised at least 50percent in step (b) to at least 50,000 cps.
 52. A method in accordancewith claim 28 wherein said viscosity is raised in step (b) to betweenapproximately 50,000 to 3,000,000 cps.
 53. A method in accordance withclaim 28 wherein said viscosity is raised in step (b) to betweenapproximately 50,000 to 250,000 cps.
 54. A method in accordance withclaim 28 wherein said impregnation bath formed in step (b) possesses aBrookfield Yield Value above the minimum value required to permanentlysuspend the largest particles of said thermosetting resin present insaid bath.
 55. A method in accordance with claim 28 wherein saidimpregnation of step (c) is carried out while said reinforcing filamentsare immersed in said bath and said work is applied as said filamentswhile under tension are passed in contact with at least one solidmember.
 56. A method in accordance with claim 28 wherein saidimpregnation step (c) is carried out by passing said substantiallyparallel reinforcing filaments in contact with the outer surface of atleast one perforated tube through which said bath is forced.
 57. Amethod in accordance with claim 28 wherein in step (d) saidconcentration of aqueous medium in said resulting fibrous material iscontrolled within the range of approximately 10 to 70 percent by weight.58. A method in accordance with claim 28 wherein in step (d) saidresulting fibrous material is dried to remove a portion of the aqueousmedium.
 59. A method in accordance with claim 28 wherein in step (d)said resulting fibrous material is dried to remove at least a portion ofthe aqueous medium and additional aqueous medium subsequently is appliedthereto in order to maintain the recited characteristics.
 60. A methodin accordance with claim 28 wherein the product of step (d) containssaid particles of matrix-forming thermosetting resin in a concentrationof approximately 6 to 45 percent by weight.
 61. A method for theproduction of an improved fibrous material suitable for the formation ofa substantially void-free composite article comprising a plurality ofadjoining substantially parallel reinforcing filaments in associationwith a matrix-forming thermosetting resin comprising:(a) providing aplurality of reinforcing fibrous tows each comprising a plurality ofadjoining substantially parallel filaments, (b) preparing a dispersionof solid particles of thermosetting resin in an aqueous medium whichcontains an effective amount of dissolved polyacrylic acid binding agentpossessing a cross-linked molecular structure, (c) raising the pH ofsaid aqueous medium to form an improved impregnation bath wherein theviscosity of the resulting bath is substantially increased to at least50,000 cps. through the stiffening of the molecules of said bindingagent and said impregnation bath has a plastic flow characteristic withshear-thinning behavior which is sufficient to substantially uniformlysuspend said particulate thermosetting resin within said bath, (d)situating said resulting bath within an impregnation apparatus, (e)aligning said reinforcing fibrous tows in a side-by-side relationship toform a substantially uniform sheet-like tape, (f) feeding saidsheet-like tape to said impregnation apparatus, (g) impregnating saidsubstantially uniform sheet-like tape with said bath while present insaid impregnation apparatus under conditions wherein said bath is causedto flow between said adjoining filaments of said sheet-like tape by theapplication of work wherein said flow inherently results in a reductionof the viscosity of said bath which aids in the incorporation of saidparticulate thermosetting resin between adjoining filaments, and (h)controlling the content of said aqueous medium in the resultingsheet-like tape to provide a product having said particles of saidmatrix-forming thermosetting resin substantially uniformly dispersedbetween adjoining filaments in the absence of fusion bonding whichinherently (1) is drapable and tacky at ambient conditions, (2) ishandleable without segregation of said particles, and (3) which upon theapplication of heat and pressure can be transformed into a substantiallyvoid-free fiber-reinforced composite article of a predeterminedconfiguration wherein said thermosetting resin becomes substantiallycompletely cured and forms the matrix phase.
 62. A method in accordancewith claim 61 wherein said reinforcing fibrous tows are selected fromthe group consisting of carbon, glass, aramid, silicon carbide, siliconnitride, boron nitride, and mixtures of the foregoing.
 63. A method inaccordance with claim 61 wherein said reinforcing fibrous tows comprisecarbon filaments.
 64. A method in accordance with claim 61 wherein saidreinforcing fibrous tows comprise glass filaments.
 65. A method inaccordance with claim 61 wherein said solid particles of thermosettingresin are selected from the group consisting of phenolic resins,polyester resins, melamine-formaldehyde resins, urea-formaldehyderesins, casein-formaldehyde resins, polyimide resins, polyurethaneresins, epoxy resins, diallyl phthalate resins, vinyl ester resins,polybutadiene(1,2) resins, cyanate ester resins, and cyanamide resins.66. A method in accordance with claim 61 wherein said solid particles ofthermosetting resin are polyimide resin.
 67. A method in accordance withclaim 66 wherein said solid particles of thermosetting resin are areverse Diels-Alder polyimide resin which is substantially fullyimidized and is capable of undergoing an addition cross-linking reactionin the substantial absence of the generation of volatile by-products.68. A method in accordance with claim 66 wherein said particles ofthermosetting resin are poly(bismaleimide) resin.
 69. A method inaccordance with claim 61 wherein said dissolved polyacrylic acid bindingagent possessing a cross-linked molecular structure is provided in step(a) in a concentration of approximately 0.01 to 2 percent by weightbased upon the total weight of the dispersion.
 70. A method inaccordance with claim 61 wherein said dissolved polyacrylic acid bindingagent is cross-linked with polyalkenyl polyether.
 71. A method inaccordance with claim 61 wherein said dispersion of step (b) possesses apH of approximately 2.5 to 3.5.
 72. A method in accordance with claim 61wherein the viscosity of said dispersion is increased in step (c)through the addition of a base which is dissolved in an aqueous solvent.73. A method in accordance with claim 61 wherein the viscosity of saiddispersion is increased in step (c) through the addition of a baseselected from the group consisting of ammonia, an alkyl amine having aboiling point less than 100° C., and mixtures of the foregoing.
 74. Amethod in accordance with claim 61 wherein the viscosity of saiddispersion is increased in step (c) through the addition of an aqueoussolution of ammonia.
 75. A method in accordance with claim 61 whereinthe viscosity of said dispersion is increased in step (c) through theaddition of an aqueous solution of a base selected form the groupconsisting of methylamine, ethylamine, triethylamine, and mixtures ofthe foregoing.
 76. A method in accordance with claim 61 wherein in step(c) the pH is raised to approximately 4 to
 10. 77. A method inaccordance with claim 61 wherein said solid particles of saidthermosetting resin are provided in said dispersion of step (b) in aconcentration of approximately 5 to 50 percent by weight based upon thetotal weight of the dispersion.
 78. A method in accordance with claim 61wherein said dispersion provided in step (b) additionally includes asurfactant in a minor concentration to aid in the wetting of saidparticles of said thermosetting resin.
 79. A method in accordance withclaim 61 wherein said viscosity is raised at least 50 percent in step(c) to at least 50,000 cps.
 80. A method in accordance with claim 61wherein said viscosity is raised in step (c) to between approximately50,000 to 3,000,000 cps.
 81. A method in accordance with claim 61wherein said viscosity is raised in step (c) to between approximately50,000 to 250,000 cps.
 82. A method in accordance with claim 61 whereinsaid impregnation bath formed in step (c) possesses a Brookfield YieldValue above the minimum value required to permanently suspend thelargest particles of said thermosetting resin present in said bath. 83.A method in accordance with claim 61 wherein said impregnation of step(g) is carried out while said substantially uniform sheet-like tape isimmersed in said bath and said work is applied as said filaments whileunder tension are passed in contact with at least one solid member. 84.A method in accordance with claim 61 wherein said impregnation step (g)is carried out by passing said substantially uniform sheet-like tape incontact with the outer surface of at least one perforated tube throughwhich said bath is forced.
 85. A method in accordance with claim 61wherein in step (h) said concentration of aqueous medium in saidresulting sheet-like tape is controlled within the range ofapproximately 10 to 70 percent by weight.
 86. A method in accordancewith claim 61 wherein in step (h) said resulting sheet-like tape isdried to remove a portion of the aqueous medium.
 87. A method inaccordance with claim 61 wherein in step (h) said resulting sheet-liketape is dried to remove at least a portion of the aqueous medium andadditional aqueous medium subsequently is applied thereto in order tomaintain the recited characteristics.
 88. A method in accordance withclaim 61 wherein the product of step (h) contains said particles ofmatrix-forming thermosetting resin in a concentration of approximately 6to 45 percent by weight.
 89. A method in accordance with claim 1 whichfurther comprises the step of applying an adhesive to the impregnatedfibrous material following step (d).
 90. A method in accordance withclaim 28 which further comprises the step of applying an adhesive to theimpregnated fibrous material following step (d).
 91. A method inaccordance with claim 61 which further comprises the step of applying anadhesive to the impregnated sheet-like tape following step (h).